The present invention relates to a process making it possible to radially follow or track a track of an optical data carrier, particularly in the form of a disk, using a light energy beam, as well as the focusing of this beam on a given plane of the carrier. The invention also relates to a device making it possible to perform this process.
Numerous processes for radially following tracks are known from the prior art. When it is a question of recording information in sequential form, e.g. video information, generally the tracks are not materialized beforehand and are instead produced in real time at the moment of recording. The information is recorded along tracks in the form of a single spiral extending from the peripheral area of the disk to a central area or vice versa, or in the form of concentric circles centred on the rotation axis of the disk. In the simplest systems, the accuracy of the recording head advance means are a prerequisite for producing this track. During reading, information recorded e.g. in the form of microreliefs, interferes with a reading beam focused in the plane of the recording face of the disk. The passage of these microreliefs beneath the focusing spot modulates the beam and this modulation is detected with the aid of photoelectric cells, which convert the light intensity variations into electrical signals, which can also be used for the purpose of following the tracks.
The aforementioned process requires a very high degree of mechanical stability of the advance of the recording head, in order to prevent an overlap of two successive grooves or at least become difficult to distinguish during reading. In order to improve the system described hereinbefore, U.S. Pat. No. 4,275,275 proposes a process using the last track recorded or one of the previously recorded tracks as a reference. A system for the optical deflection of the recording spot is added to the mechanical advance system of the recording head. The reading spot is made to follow an already recorded groove of the track by a conventional position control. With the aid of the optical deflection system, the recording spot is kept at a constant distance from the reading spot, which is equal to an integral multiple of the pitch of the track.
However, when it is desired to record information in a random manner, for example in informatics application, it is no longer possible to use the aforementioned processes or similar processes. It is generally necessary to materialize beforehand the tracks before which the information can be recorded. For this purpose it is conventional practice to produce a preetching in some form. According to an embodiment described in U.S. Pat. Nos. 4,252,889, 4,288,510 and 4,334,007 during the production of the carrier, the tracks are materialized in the form of a smooth groove made in an auxiliary coating of said carrier. These tracks can be detected even in the absence of any information recording, the latter being carried out in a subsequent phase in a photosensitive or thermosensitive coating in contact with the auxiliary coating.
In a preferred variant of the prior art, the preetched tracks can coincide with the regions in which the information is recorded, which leads to a so-called single-track system.
According to other processes, the preetched track or tracks are separate from the tracks along which the information is recorded, giving so-called two-track systems. In order to discriminate these two different type of track, it is possible to proceed in such a way that the preetching consists of a signal which can be broken down into a first frequency spectrum, whilst the information can be broken down into a second separate frequency spectrum. During writing, it is possible to use the reading beam for following the preetched track, in accordance with aforementioned U.S. Patents.
The main disadvantage of the process described hereinbefore is that it does not permit a maximum recording density, because it requires a minimum of one supplementary preetched track for one preetched information track. In addition, it requires the use of two beams, one for the radial tracking of the preetched track and the other for the writing or reading of information on the track used for recording.
In addition, single-track carriers having a preetching are also not free from disadvantages. They generally require the use of two beams, one for recording and the other for radial tracking. In addition, even if the preetched track can easily be distinguished from the remainder of the disk (intertrack areas) when no information is recorded, this does not apply when information has been recorded. Thus, if precautions are not taken, there can be inversions of contrasts leading to tracking errors.
To obviate these disadvantages, French Patent Application 82 04 214 proposes a radial tracking device utilizing a data carrier having a preetching used for the radial following of the track constituted solely by flags, which are regularly or non-regularly distributed along the tracks.
According to this French application, the preetching is constituted by a sequence of discrete non-contiguous elements materializing the mean axis of the tracks. The spatial distribution of these discrete elements may or may not be uniform. According to a first variant, each discrete element is constituted by a smooth track portion. According to a second variant, each discrete element has several portions defining the special code. According to supplementary variants, each element has, apart from a portion centred on the mean axis of the tracks, one or more portions which are displaced with respect to the mean axis. Finally, each of the preetchings can be preceded by an auxiliary preetching element used for synchronization purposes. The track following device comprises photodetectors, measuring circuits incorporating sampling--inhibiting means or memory-possessing integrators, as well as sampling circuits. A track radial following error signal is processed on the basis of the evolution of signals detected during successive passages in an illuminated area of the disk surface by a track following spot.
Apart from the correct radial tracking ensured by the preferred process described hereinbefore, a second requirement linked with the reading or writing of information on an optical carrier, involves the focusing of the light energy beam or beams used, which must be focused on the carrier at a spot, which must permanently follow possible fluctuations of the level of the tracks.
In order to obtain this, it is known that the correct focusing of the beam is most frequently obtained by means of a focusing control device having a feedback loop, which maintains the distance between the optical device and the etched surface of the carrier completely constant. The synchronization and maintainance of the control device are obtained by detecting fluctuations of an electrical signal which, according to a first variant, is derived from reading the recorded information and, in a second variant, from the detection of the preetching materializing the tracks, e.g. in the form of a smooth groove. Thus, the same difficulties as referred to hereinbefore in connection with radial tracking are encountered.